fib-rs

A highly optimized Fibonacci number calculator for Rust that efficiently computes arbitrarily large Fibonacci numbers.

Features

Fast doubling algorithm: Calculates Fibonacci numbers in O(log n) time
Handles massive inputs: Compute Fibonacci numbers up to F(10,000,000) and beyond
Automatic optimization: Uses primitive integers for smaller inputs and BigUint for larger ones
CLI application: Simple command-line interface for quick calculations
Web API: HTTP server with JSON interface for network-based calculations

Installation

cargo add fib-rs

Usage

As a library

use fib_rs::fib;

// Calculate F(1000)
let input = 1000;
let result = fib_rs::fib(input);
println!("F({}) = {}", input, result);

Command-line application

# Install only the CLI (this is the default)
cargo install fib-rs

# Or explicitly specify the CLI feature
cargo install fib-rs --features="cli" --no-default-features

# Calculate the 100th Fibonacci number
fib 100

Web server

# Install only the web server
cargo install fib-rs --features="web" --no-default-features

# Run the server (defaults to port 8080)
fib-web

# Or specify a custom port with an environment variable
PORT=3000 fib-web

Then send a POST request:

curl --request POST \
     --url "http://localhost:8080/fib" \
     --header "Content-Type: application/json" \
     --data '{"n": 100}'

Response:

{"F":"354224848179261915075"}

Installation Options

The package provides these installation options:

--features="cli": Install only the command-line interface (default)
--features="web": Install only the web server
--features="cli,web": Install both CLI and web server
--no-default-features: Disable default features (use with other feature flags)

Performance

The implementation uses a fast doubling algorithm with the following optimizations:

For n ≤ 185: Uses primitive u128 integers
For n > 185: Uses BigUint for arbitrary precision

Benchmark results:

Specifications:

15-inch MacBook Air (2025)
M4 Chip, 10C CPU, 10C GPU
32GB Unified Memory
macOS Sequoia 15.4

Input Size	Computation Time
F(1,000)	~876ns
F(10,000)	~8μs
F(100,000)	~10ms
F(1,000,000)	~329ms

Algorithm Details

This implementation uses the fast doubling algorithm for Fibonacci numbers, which has logarithmic time complexity:

For even n: F(2k) = F(k) (2F(k+1) - F(k)) For odd n: F(2k+1) = F(k+1)^2 + F(k)^2

This approach is vastly more efficient than naive recursive or iterative methods for large inputs.

License

This project is licensed under the MIT License.

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

fib-rs 0.1.0